Problems in Joining two sculpted object
You should use voxel remesh in blender 2.81, it is quite good for joining two separate object. First join the head and the body using Ctrl-J, then go to Object Data tab in the properties panel and under Remesh click Voxel Remesh. 



More information:
https://docs.blender.org/manual/en/latest/modeling/meshes/editing/retopology.html